A number of recent studies (see, for example, Sun et al. (2000) J. Am. Chem. Soc. 122:12340-12345; Miller et al. (2001) JACS 123:12335-12342; Harrell et al. (2003) Anal. Chem. 75:6861-6867; Cooper et al. (2004) Nano Lett. 4:377-381; and Hinds et al. (2004) Science 303:62-65) have focused on the development of membranes with nanometer-scale pores. Such membranes could find application in the area of size-based chemical and biological separations, provided pore sizes can be reduced to molecular diameters and a high flux of the permeate molecule can be achieved. On this size scale, however, little is known about the behavior of fluids; deviations from continuum transport can occur as the channel size becomes comparable to molecular diameters. For example, it is known that the intra-pore diffusivity (as compared with the bulk diffusivity) decreases when the pore size becomes comparable to molecular dimensions (see for example, Deen (1987) AIChE J. 33:1409-1425). In electrolyte solutions, if the pore radius is comparable to the Debye length (electrical double layer thickness), a situation can arise where the ion concentration within the pore exceeds that of the bulk solution (see, for example, Cerver et al. (2001) J. Membrane Sci. 191:179-187), which may have some interesting consequences for the development of ion-selective membranes.
Carbon nanotubes, with inner diameters as small as 1 nm, were postulated to provide an ideal system for the study of confined molecular transport. A number of recent molecular dynamics simulations have focused on transport within these materials (see for example Koga et al. (2001) Nature 412:802-805; Hummer et al. (2001) Nature 414:188-190 and Gao et al. (2003) Nano Lett. 3:471-473). Many exotic predictions have been made, from the formation of novel phases of ice (see for example Koga et al. (2001) supra) and pulsed one-dimensional water transport (see for example Hummer et al. (2001) supra), to the spontaneous insertion of ss-DNA into a single wall carbon nanotube (see for example Gao et al. (2003) supra). What has been lacking, however, is an experimental platform for experimental verification of these and other predictions.
One method for fabrication of such a platform involves chemical vapor deposition of carbon within the pores of an alumina membrane (see for example Miller et al. (2001) supra). Typically, the pore sizes achieved by this approach (of order 100 nm) are larger than the size range of interest for chemical and biological separations. In addition, the inner walls of these carbon nanotubules are only semigraphitic and thus do not possess the inherent smoothness of a purely graphitic carbon nanotube (CNT). It is also reported (see Miller et al. (2001) supra) that tubes prepared in this manner possess acidic surface sites (—COOH) on their walls. The inherent smoothness and inertness of a purely graphitic CNT are the attributes that, according to the molecular dynamics simulations (Skoulidas (2002) Phys. Rev. Lett. 89:185901-1-185901-4), give rise to a high molecular flux through CNTs.
Another method for preparation of a nanotube membrane involves embedding an amorphous carbon coated graphitic tube in an epoxy matrix (Sun et al. (2000) supra). However, the resultant pore diameter in these materials is again large (about 150 Nm). A method that has managed to produce membranes in the single nanometer size regime involves creating damage tracks in a polycarbonate film by use of a collimated fission fragment beam, followed by etching in basic solution (Harrell et al. (2003) supra). With subsequent electroless plating, single gold nanotubes of order 2 nm were prepared.
More recently, a polystyrene-coated CNT membrane was fabricated (Hinds et al. (2004) supra). The pore sizes of this membrane are reported to be consistent with that of multiwall CNT inner diameters (about 7.5 nm). However, extremely small carbon nanotubes, that can be fabricated into an array or membrane, were not heretofore reported.
Several simulations of water transport through single walled carbon nanotubes (SWCNTs) have suggested that water not only occupies these channels, but also that fast molecular transport takes place, far in excess of what continuum hydrodynamic theories would predict if applied on this length scale (Hummer et al. (2001) supra and Kalra et al. (2003) Proc. Natl. Acad. Sci. USA, 100, 10175). Molecular dynamics (MD) simulations attribute this enhancement to the atomic smoothness of the nanotube surface and to molecular ordering phenomena that may occur on confined length scale in the 1-2 nm range (Hummer et al. (2001) supra and Kalra et al. (2003) supra). For similar reasons, simulations of gas transport through SWCNTs (Skoulidas et al. (2002) Phys. Rev. Lett. 89, 185901) predicted flux enhancements of several orders of magnitude relative to other similarly-sized, nanoporous materials. Membrane-based gas separations, such as those utilizing zeolites (Lai et al. (2003) Science 300, 456), provided precise separation and size exclusion, although often at the expense of throughput or flux. A SWCNT membrane may offer the highly selective, high flux membrane that, prior to Applicants' disclosure, currently did not exist in the field.
Researchers have recently fabricated multi-walled carbon nanotube (MWCNT) membranes with larger pore diameters (6-7 nm) having vertically aligned arrays of MWCNTs (Hinds et al. (2004) supra) and by templated growth within nanochannel alumina (Li et al. (1999) Appl. Phys Lett. 75:367). Quantifying transport through an individual tube in a MWCNT membrane is difficult, however, as MWCNTs are prone to blockages, in particular by “bamboo” structures and catalyst particles that can migrate to and obstruct the nanotube interior (Cui et al. (2000) J. Appl. Phys. 88, 6072). The consequence of such blockages is a significant reduction of the active membrane pore density. In contrast, there are few, if any, reports of “bamboo” structure formation or catalyst migration for SWCNT or double-wall carbon nanotubes (DWCNTs). However, several groups have reported that it is difficult to produce vertically aligned carbon nanotubes of this size (Hata et al. (2004) Science 306:1362). No admission is made that any reference cited in this section or any other section herein is admitted prior art.